Vacuum-actuated conveyor tables enable sheets to be conveyed in an underlapped arrangement to printing machines and the like. One of the advantages of this arrangement is that the top of the conveyor table is not obstructed by any devices with retaining elements. Consequently, the sheets are readily accessible on the conveyor table in the event of interruptions in the operation. Also, there is no risk of accident and there is no need for any of the elements of the frame to be adjusted.
A method and apparatus of this kind are shown in German Pat. No. 713 529 which describes a sheet conveyor device, and more particularly sheet feeders for letter-press and similar machines. In this specification, endless belts formed with suction ports run over vacuum slots in the conveyor table. Since air is extracted through the slots, sheets of paper are held and conveyed on the perforate belts. A disadvantage of the disclosed arrangement is the low volume of the slots and the provision of a plunger-type vacuum pump. Consequently, the transport with this system is not reliable. Also, relatively high vacuums are required to obtain any substantial transport effect and this results in increased friction of the belts on the conveyor table.
Certain of the disadvantages of the aforementioned apparatus are avoided in a device for automatically loading moistened veneer sheets in a belt or drum dryer and for unloading and stacking the dried veneer sheets, as described in German Offenlegungsschrift No. 2 523 482. In this device, the perforate conveyor belts run on the underside of a large volume vacuum box parallel to its surface, which is formed with apertures therein. On the opposite side the box is provided with a fan as a vacuum source whereby a generally uniformly distributed vacuum is generated over the entire area of the underside of the box and a vacuum storage effect is obtained as a result of the large box volume. A serious disadvantage of this device, however, is that it is impossible to obtain any controlled and accurately adjusted conveyance of imbricated paper sheets per unit of time, particularly since it is designed as a single sheet conveyor. More, particularly, there is no provision for controlling the further conveyance of the individual sheets in the area where they are transferred to the alignment means of a processing machine, which in this case is the sheet dryer. In this regard, the sheet is held fast to the end, without any need for consideration of the position or accurate arrival of the sheet per unit of time.
Other devices have, however, been disclosed to enable the sheet to be aligned. For example, German Pat. No. 836 355 describes a sheet feeder device having additional suction means at the end of the conveyor table operating with conveyor belts. The vacuum means are in the form of vacuum rollers and act from the top on the end of the sheet being conveyed to the front gauges. A disadvantage of this system is that the rollers have to be controlled per unit of time both in respect of their movement and vacuum applied to them. In these conditions it is impossible to ensure that the sheet will not be conveyed too far and bulge at the middle, since it is being held at the front gauges. On other occasions, the next sheet could also be engaged by the vacuum rollers and be conveyed in an uncontrolled manner. Moreover, the complete apparatus has to be accurately adjusted to the sheet end and thus blocks access to the sheet on the feed table.
Another apparatus of this kind is described in German Pat. No. 1 152 707. The sheet deceleration and alignment device described in this patent comprises left and right vacuum chambers in the form of vacuum rollers or vacuum boxes having perforate conveyor belts extending therearound disposed at the conveyor table and adapted to decelerate and align the sheet against the front gauges. The speed of the transport conveyor belts is below the speed of the conveyor table belts and are also disposed adjacent the conveyor table and before the printing machine feed table. They are, therefore, a considerable distance from the front gauges and their action is subject to the risk that the sheet will buckle as it is held against the front gauge due to the conveying effect of the alignment means. While the disadvantage of the previously described device (i.e., alignment means provided above the feed table) is admittedly avoided, the disadvantage in terms of the controls required still exists. Also, the working speed of this feed device is reduced as a result of the reduced transport speed of the additional alignment means.